This is from an American journal in 1807, published in Philadelphia. I draw attention to the discussion of charring barrels, at pg. 45.
This discussion summarizes research in France a few years earlier by Bertholet, a French scientist. He advised to char barrels to keep water fresh on sea voyages. He also advised to use the process to improve wines and spirits, which advice is repeated in the attached link. The experiments with charcoal in general as a purifier started apparently with Lovitz, another European scientist working in the later 1700's.
Charred barrel aging for bourbon became typical around this period or shortly after.
Does this mean artisans hadn't hit on the process themselves earlier? No, but I rather think that word got around from scientific circles that spirits were improved in this way and this kick-started the aging of bourbon and straight rye in charred wood, at least for the best qualities.
Why didn't scotch end up being aged in this same way? I theorise there wasn't as much cheap wood in Europe even then, they needed to reuse casks.